The total thickness and number of layers of multilayer PCB board are limited by PCB board characteristics. The designer must consider the limitations of the characteristics and processing technology of PCB board in the process of PCB design.
The parameters that have the greatest influence on PCB design and machining are dielectric constant and loss factor. For the design of PCB multilayer board, the selection of board material also needs to consider machining punching, lamination performance and so on. Based on the characteristics of transmission line impedance, loss, transmission wavelength analysis and material comparison, product design must consider the cost, market factors.
Therefore, it is suggested that in PCB design designers should consider the following key factors when selecting PCB materials:
(1) different signal operating frequencies require different materials.
(2) FR4 can be used for PCB working below 1GHz, with low cost and mature multi-layer pressing plate technology.For example, if the signal input and output impedance is low (50 ohm), the characteristic impedance of the transmission line and the coupling between lines should be strictly considered when lining. The disadvantages are that FR4 materials are different by different manufacturers and batches, and the dielectric constant is different (4.2-5.4) and unstable.
(3) for optical fiber communication products working at more than 622Mb/s and small signal microwave transceivers with more than 1G and less than 3GHz, modified epoxy resin materials such as S1139 can be selected, because their dielectric constants are relatively stable at 10GHz, the cost is low, and the multi-layer pressing pcb technology is the same as FR4. For example, 622Mb/s data multiplexing branch, clock extraction, small signal amplification, optical transceiver, etc. are recommended to use this kind of materials, so as to facilitate the production of multilayer PCB,while pcb cost slightly higher than FR4, the disadvantage is that the substrate thickness is not as complete as FR4 varieties. Or adopt RO4000 series such as RO4350, but at present domestic commonly uses RO4350 double-sided PCB. The disadvantage is: these two kinds different materials thickness variety quantity is not complete, because pcb thickness size request, is not convenient to make the multilayer printed circuit board. For RO4350, the specification that board material manufacturer produces 10mil/20mil/30mil/60mil 4 kinds of boards thick, and at present in domestic there is less import materials, restricted PCB board design accordingly.
(4) for large signal microwave circuits below 3GHz, such as power amplifiers and low-noise amplifiers, it is recommended to choose double-sided materials similar to RO4350. RO4350 has a fairly stable dielectric constant, low loss factor, good heat resistance, and processing technology similar to FR4.Its material cost is slightly higher than FR4.
(5) microwave circuits above 10GHz, such as power amplifiers, low-noise amplifiers and up-down frequency converters, have higher requirements on plate materials. It is recommended to use double-sided plates with the performance equivalent to Fr4.
(6) the multi-layer PCB board of wireless mobile phone requires the dielectric constant stability, low loss factor, low cost and high media shielding requirements of the board. It is recommended to choose the board with similar performance PTFE (multi-purpose in the United States/Europe), or the combination of FR4 and high-frequency board bonding to form a low-cost and high-performance laminate board materials.